Existing wireless system architectural configurations impose stringent constraints on the system designer with regards to transmitting communication signals. Moreover, such configurations often provide low reliability communication links, high operating costs, and an undesirably low level of integration with other system components.
In the radio frequency (RF) section of a conventional low-cost wireless transmitter configured with analog components, a considerable level of distortion occurs when RF signals are processed. Higher cost components with better distortion characteristics that enhance signal quality may be overlooked during the design phase in order to reduce the cost of the end-product.
For example, a common problem associated with conventional wireless communication systems is that a local oscillator (LO) signal, used for the modulator of an analog radio transmitter, leaks to the output of an RF power amplifier in the transmitter. This LO leakage is caused by suboptimal isolation of the LO with respect to analog circuit components in the transmitter, as well as an unavoidable direct current (DC) offset voltage at the mixer stage of the modulator due to mismatches and imperfections in the analog circuit components.
There are several reasons for suppressing carrier leakage. Technical specifications for many mobile communication systems, including universal mobile telecommunications system (UMTS), define a mask which sets forth the maximum permissible radiated signal energy as a function of frequency. Radiated signals that do not fall within the mask may violate existing Federal Communications Commission (FCC) or other rules governing spurious emissions, and may disrupt services provided by the communication systems. Moreover, an unwanted carrier component at the output of the RF power amplifier can degrade the linearity of the RF power amplifier, due to the presence of increased signal levels.
In transmitters and transceivers that employ heterodyne circuitry, LO feedthrough can be dramatically reduced through the use of proper frequency planning and filtering. However, in broadband systems, the RF and LO bands often overlap. For homodyne transmitters, the RF and LO bands virtually always overlap.
There are several current methods which exist to suppress LO leakage, including those that utilize RF parameter balancing techniques. However, practical component tolerances rarely permit more than approximately 30 dB of carrier leakage suppression.
Another technique of adding a correctly leveled and phase-shifted part of the LO signal to the RF signal has the disadvantage of requiring very complex mixers.
Because the costs of components that process RF analog signals are higher than the components that use DSP, it is desired to provide a digital baseband (DBB) system, including a low cost transmitter with low noise and minimal power requirements, that utilizes DSP techniques to provide an improved and less complex method and system for suppressing carrier leakage in an analog radio transmitter.